Investigators searching clandestine graves have found 32 bodies and nine human heads in a municipality in southern Mexico where rival drug gangs have been engaged in a wave of extortion, kidnappings and turf battles, authorities said on Thursday.

Soldiers and police found the graves on Tuesday at an outlaw camp in Guerrero state after receiving a tip that people were being held at the site located near a mountain in the municipality of Zitlala. They said they rescued a kidnap victim and discovered 12 bodies and human remains in coolers.

On Thursday, officials announced that further excavations of the site had found a total of 32 bodies and nine human heads.

Roberto Alvarez Heredia, spokesman for the Guerrero coordinating group, said soldiers were combing the area to see if there were any more clandestine graves. Investigators were working to identify the bodies and the killers. Drug gangs frequently decapitate their victims.

Residents of the community of Tixtla, Guerrero found nine decapitated bodies on Monday along a highway. Prosecutors are looking into whether the nine heads found in Zitlala correspond to these bodies.

Guerrero has seen an upsurge in gang-related violence. The government announced on Monday that it is stepping up the use of joint police-army patrols in areas known to be particularly violent.

The largely rural, impoverished state had 1,832 reported homicides in the first 10 months of 2016. If that rate continues unabated, Guerrero would be on track to have a homicide rate of about 60 per 100,000.

That would rival the recent peak year of violence in the state, in 2012, when there were about 68 homicides per 100,000 inhabitants.